Talks

Invertible disformal transformations are a useful tool to investigate ghost-free scalar-tensor theories. By performing a higher-derivative generalization of the invertible disformal transformation on Horndeski theories, we construct a novel class of ghost-free scalar-tensor theories, which we dub generalized disformal Horndeski theories. In the talk, I will clarify the basic idea for constructing the invertible disformal transformation with higher derivatives. I will also discuss some aspects of the generalized disformal Horndeski theories, including the consistency of matter coupling and cosmological perturbations.

Gravitational collapse shaped the cosmic large-scale structure and created a plethora of different density environments. For optimally probing gravity with galaxy surveys like Euclid and Rubin LSST, we need to dissect different density environments that are lumped together in traditional two-point statistics. I will explain how the one-point probability distribution of dark matter densities can be predicted analytically including signatures of modified gravity that match with cosmological simulations for nDGP and f(R) gravity. I will provide an outlook on how those predictions can be translated to galaxy clustering and weak lensing and observables.

There are now multiple direct probes of the region near black hole horizons, including direct imaging with the Event Horizon Telescope (EHT). As a result, it is now of considerable interest to identify what aspects of the underlying spacetime are constrained by these observations. For this purpose, we present a new formulation of an existing broad class of integrable, axisymmetric, stationary spinning black hole spacetimes, specified by four free radial functions, that makes manifest which functions are responsible for setting the location and morphology of the event horizon and ergosphere. We explore the size of the black hole shadow and high-order photon rings for polar observers, approximately appropriate for the EHT observations of M87*, finding analogous expressions to those for general spherical spacetimes. Of particular interest, we find that these are independent of the properties of the ergosphere, but does directly probe on the free function that defines the event horizon. Based on these, we extend the nonperturbative, nonparametric characterization of the gravitational implications of various near-horizon measurements to spinning spacetimes. Finally, we demonstrate this characterization for a handful of explicit alternative spacetimes.

Simulating non-linear scales of structure formation is essential to make use of frontier data from Stage IV galaxy surveys. Performing these simulations in modified gravity theories introduces additional challenges, and further forces us to make choices about which theories we deem ‘worth’ the computational investment. Horndeski Gravity is very helpful in this regard, as it encompasses a large swathe of models of major interest. I’ll introduce Hi-COLA, a software suite which simulates large-scale structure formation in the class of luminal Horndeski theories. Hi-COLA was designed to be: i) flexible — it avoids hard-coded models and instead receives a user-specified Lagrangian; ii) consistent — the background expansion history, linear growth and nonlinear screening are solved consistently with one another; iii) efficient — using the COLA method, large sets of simulations can be generated at low cost. I’ll explain how Hi-COLA can be used to make robust predictions for scalar-tensor theories on nonlinear scales. If time permits, we’ll also dip a toe into constraining the Horndeski framework with gravitational waves.

We will discuss black hole solutions in Horndeski and beyond Horndeski theories. Starting from the no hair paradigm in GR we will elaborate on one of the first black hole solutions with secondary hair. We will then start by introducing stealth solutions, in other words GR metrics endowed with a non trivial scalar field, their regularity properties, shortcomings etc. We will then go on to construct black holes with primary scalar hair which can be regular black holes and modify usual GR static metrics. We will discuss their properties and the status of explicit stationary metrics to conclude.

The ground state of a one-dimensional spin-1/2 uniform antiferromagnetic Heisenberg chain (AfHc) is a Tomonaga-Luttinger liquid which is quantum-critical with respect to applied magnetic fields upto a saturation field Hs beyond which it transforms to a fully polarised state. Wilson ratio has been predicted to be a good indicator for demarcating these phases [Phys. Rev. B 96, 220401 (2017)]. From detailed temperature and magnetic field dependent magnetisation, magnetic susceptibility and specific heat measurements in a metalorganic complex and comparisons with field theory and quantum transfer matrix method calculations, the complex was found to be an excellent realisation of a spin-1/2 AfHc. Wilson ratio obtained from experimentally obtained magnetic susceptibility and magnetic contribution of specific heat values was used to map the magnetic phase diagram of the uniform spin- 1/2 AfHc over large regions of phase space demarcating Tomonaga-Luttinger liquid, saturation field quantum cri...

Trigonal PtBi2 is a layered Van der Waals semimetal without inversion symmetry, featuring 12 Weyl points in the vicinity of the Fermi energy. We present and discuss the experimental evidence that its topological Fermi arcs superconduct at low temperatures where bulk superconductivity is absent. With first-principles calculations we investigate in detail the bulk and surface electronic structure of PtBi2, and discuss the spin texture as well as the momentum-dependent localization of the arcs. Motivated by the experimentally observed recovery of inversion symmetry under pressure or upon doping, we interpolate between the two structures and determine the energy and momentum dependence of the Weyl nodes. For deeper insights into the surface superconductivity of PtBi2, we present a symmetry-adapted effective four-band model that accurately reproduces the Weyl points of PtBi2. We supplement this model with an analysis of the symmetry-allowed pairings between the Fermi arcs, which naturally m...